Glass-like thermal conductivity in nanostructures of a complex anisotropic crystal

Size effects on vibrational modes in complex crystals remain largely unexplored, despite their importance in a variety of electronic and energy conversion technologies. Enabled by advances in a four-probe thermal transport measurement method, we report the observation of glass-like thermal conductivity in $\ensuremath{\sim}20$-nm-thick single crystalline ribbons of higher manganese silicide, a complex, anisotropic crystal with a $\ensuremath{\sim}10$-nm-scale lattice constant along the incommensurate $c$ axis. The boundary-scattering effect is strong for many vibrational modes because of a strong anisotropy in their group velocities or diffusive nature, while confinement effects are pronounced for acoustic modes with long wavelengths along the $c$ axis. Furthermore, the transport of the nonpropagating, diffusive modes is suppressed in the nanostructures by the increased incommensurability between the two substructures as a result of the unusual composition of the nanostructure samples. These unique effects point to diverse approaches to suppressing the lattice thermal conductivity in complex materials.